It is curious that so many pilots have so many misconceptions about the V-22 crash in Marana (Yuma), Arizona in April 2002. Let's address just two of them.
1. Many pilots believe that Nighthawk 72 intentionally initiated a RoD of 2500+ fpm, and that this is what "caused" the aircraft to enter Asymm-VRS. This is what the Marines and Bell/Boeing would like you to believe without question. Trouble is, it's nonsense all around. If you look at the descent profile taken from the FDR, you'll see that as the formation came down, Nighthawk 72 (the second ship - the one that crashed and burned)
did have a momentary RoD of 3945 fpm as they passed through 820 feet. Suicide! Right? Well...maybe not. A mere nine seconds later, as the ship passed through 560 feet, the RoD was down to 150 fpm, still under full control and with the nacelles at 90 degrees. During this time, the aircraft was decellerating from 101 knots to 52 knots at 3.3 kts/sec (i.e. they were putting on the brakes hard!).
At 500 feet, they were only coming down at 800 fpm and still doing a healthy 52 knots of
airspeed - in other words, well within the U.S. NATOPS parameters for "safe" rotary-wing flight.
It was at this point that the PIC of the accident a/c pulled his nacelles back to 95 degrees and everything came apart. His RoD shot to 2083 fpm at 350 feet (yet his airspeed was still 41 knots), then 2247 fpm at 339 feet (40 knots). At that point they were only six seconds away from dying.
So. Do we really believe that Major Brow
intentionally initiated a 2083 fpm RoD when he was only 350 feet above the ground? Personally, I do not. I believe that this "incredible" rate of descent was the
result of the aircraft entering Asymm-VRS, rather than the
cause of it. This Asymm-VRS was probably excited when the ship was around 500 feet AGL with the nose up and the nacelles tilted back to 95 degrees as Major Brow desperately tried to slow down, maintain position on Lead and salvage the approach.
In the end, remember, high RoD does not cause VRS. It is only when you try to recover from that high RoD by yanking in a bunch of power at
very low airspeed (we're talking below ETL here) that VRS can be excited. From the crash stats, Nighthawk 72 never got below 40 knots of
airspeed at any time during the approach.
2. Then GLSNightPilot weighs in with this bit:
I'm still not convinced of the accidents were caused by asymmetric VRS, it could just as easily been a rate of descent too high to stop before hitting the ground, especially the one which involved formation flight. Lead just let things get out of hand, & his wingman paid the price.
See what I mean? Not to pick on him but there are probably many pilots who still don't even believe that Nighthawk 72 crashed because of Asymm-VRS. However it must be noted that the Osprey did not hit the ground in a level flight attitude. On the contrary, it crashed inverted. And if so, then the aircraft departed controlled flight at some point. It points out to me how little people really know about the accident in Marana. I could write a thesis on it. And I probably will.
The thing about VRS is that it is capricious. We have general guidelines, but one cannot say with certainty that you
WILL get into VRS at "this" RoD and "this" power-setting (we will assume that A/S is zero because that is a requirement). Conversely, one cannot say with certainty that you will not.
All we know are the boundaries within which VRS has been demonstrated and/or experienced in the past. (And that's the key. Past peformance is no guarantee of future results.) For a helicopter, those parameters are:
a) low A/S (below ETL);
b) greater than 300fpm Rod; and
c) power applied (i.e. not in autorotation).
Unfortunately, Mother Nature does not like it when we try to make her conform to our "rules," no matter how logically and scientifically well thought-out they are. Or we
think they are. Strangely, there are many pilots who've successfully flown deep in the area of the flight envelope defined by the three areas above. No crash! How 'bout dat? Why did they
not get into VRS? Like I said, it's capricious.
The two pilots in Nighthawk 72 were highly experienced aviators - they just didn't have "much" helo time. I think about this and ponder the fate of the low-time crews that will be flying the operational tiltrotors if and when they are accepted into the fleet for unrestricted use.
God help them.